Conventional positron emission tomography (PET) scanning relies on the coincident detection of anti-parallel 511 keV gamma rays that arises when anti-matter (a positron) and matter (an electron) annihilate each other. Coincident detection of these anti-parallel gamma rays is accomplished using a stationary ring of detector elements, and various computer algorithms are used to reconstruct the original distribution of isotope. Relative to single photon emission computed tomography (SPECT), PET is dramatically more sensitive (˜1% for most clinical PET scanner vs. 0.01% for clinical SPECT scanners) owing in large part to the absence of a collimator. PET also has higher resolution than SPECT, with most clinical scanners providing final reconstructed voxel sizes of 8 mm×8 mm×8 mm (0.5 cm3). Since human body has no naturally occurring positrons, the administration of an appropriate positron-emitting radiotracer permits the quantitative, 3D localization of disease, as well as the study of a variety of functional processes in vivo {Raichle, 1979; Yamamoto, 1984}.
PET has revolutionized the detection and staging of human cancer, however, it is far from reaching its potential. Unfortunately, the term “PET scanning” is presently synonymous with the use of 2-[18F]fluoro-2-deoxy-D-glucose (18FDG) as the radiotracer. Although valuable as a cancer biomarker, 18FDG has variable uptake and retention in many tumors {Kelloff, 2005}, as well as high uptake in normal tissues and organs {Kumar, 2006}. In general, there remain two major problems in the field of PET cancer imaging: 1) the difficulty in producing high affinity small molecule ligands specific for particular cancers, and 2) the complex and expensive chemistry infrastructure required for traditional radiolabeling with 18F.